The invention relates to a new economic process for recovering and purifying paraxylene from a charge of hydrocarbons which contain paraxylene at a concentration that is greater than that at thermodynamic equilibrium. It relates, in particular, to the processes that enrich their effluent with paraxylene to more than 30%, particularly the processes that provide at least one paraxylene-enrichment step by crystallization at very low temperature, for example, this crystallization step being followed by a step of purifying the paraxylene in at least one stage (U.S. Pat. No. 3,177,265 and U.S. Pat. No. 2,866,833).
The invention relates in particular to a process for preparing and purifying paraxylene from a charge of aromatic hydrocarbons having 8 carbon atoms comprising a combination of steps for selective adsorption, for purification by high-temperature crystallization and for isomerization as described in the patents of the applicant FR 2,681,066 and U.S. Pat. No. 5,284,992 that are incorporated by reference.
It applies in particular to the preparation of paraxylene with very high purity for the production of terephthalic acid for the synthesis of synthetic fabrics.
Crystallization has been used commercially for a very long time to isolate and purify paraxylene, typically from a mixture of xylenes and ethylbenzene close to equilibrium (C.sub.8 -aromatic fraction).
The C.sub.8 -aromatic fractions ordinarily come from a reforming unit or from an ethylene production unit and especially from the reformates by distillation alone or extraction in combination with a distillation as a function of the composition of the charge, of the sensitivity to impurities of the downstream technologies and of the savings of the recovery processes.
The typical composition by weight of a C.sub.8 -aromatic fraction is approximately 22% paraxylene, 16% ethylbenzene, 18% orthoxylene and 44% metaxylene. Very low temperatures are generally required to effectively recover, by crystallization, the paraxylene from a C.sub.8 fraction. Furthermore, there is a eutectic limit that prevents the complete recovery of all the paraxylene from a C.sub.8 fraction. For example, in a low-temperature crystallization unit, for a C.sub.8 -aromatic fraction containing 22% weight of paraxylene, only about 50 to 65% of the paraxylene is recovered, the remaining paraxylene will be found in the paraxylene-depleted mother liquor, which can be introduced into an isomerization unit. The latter will isomerize the metaxylene, orthoxylene and in certain processes, the ethylbenzene, and a mixture of xylenes close to equilibrium containing about 22% paraxylene will again be obtained. This recycled flow, in combination with the fresh charge, is then introduced into the crystallizer so as to recover more paraxylene. In this way, the C.sub.8 -aromatics can be recycled to extinction and recovery of a maximum amount of paraxylene, with by-products resulting from the isomerization. The production of these by-products is a significant insufficiency of the system since each time that xylenes are introduced into the isomerization unit, a part of them is converted into non-xylenes such as toluene. Actually, the chemistry of the isomerization is very complex and the main reactions which lead to losses of xylenes are the disproportionation (dismutation) of the xylenes into toluene and trimethylbenzenes, dealkylation of the xylenes, and in certain cases even the formation of non-aromatics. To do this, the total yields of a set of isomerization and crystallization units are typically 60 to 80% and a large isomerization recycling loop is necessary to maximize the recovery of the paraxylene.
In the 1970's, another process was marketed to prevent the eutectic limitation from a low-temperature crystallization. This process uses an adsorption to separate the paraxylene from a mixture of xylenes. The adsorption makes it possible to recover more paraxylene from a C.sub.8 -aromatic fraction. Thus, from a charge containing 22% weight of paraxylene, it is possible to recover approximately 97% of the latter by adsorption, leaving about 1% paraxylene in the mother liquor. It is advantageous to recover the product with a greater efficiency, since this entails the use of a much smaller isomerization loop for the complete recycling of the paraxylene-depleted fraction. This offers several advantages, particularly a lower investment cost on a new unit or on the expansion of an existing unit, overall higher yields due to low losses in isomerization, and lower operating costs linked to the size of the isomerization loop.
Several drawbacks linked to the system for recovery by adsorption of paraxylene are noted, however: high investment cost, difficulty in obtaining paraxylene with very high purity, sensitivity of the adsorbent to impurities in the charge and sensitivity of the control system to changes in quality of the charge.
Furthermore, a process for crystallization that uses two separate crystallization stages (AMOCO process) has been proposed. Demand for a higher purity of paraxylene was increasingly difficult to satisfy by a single crystallization. A process was therefore developed which completely remelts the crystals that were isolated from the first crystallization stage at very low temperature. After complete remelting, the flow was cooled to about -17.degree. C. to recrystallize the paraxylene to the desired purity. This process was able to deliver high purities (99.5%+) after washing. However, it has the drawback of higher operating and investment costs due to the cost of the energy associated with a complete remelting and then to a recrystallization of the paraxylene.
A process of the applicant has recently been patented, which combines in particular an adsorption with a crystallization; it is patent U.S. Pat. No. 5,284,992 which describes a selective adsorption to recover the paraxylene from a charge containing a mixture of xylenes. The paraxylene is then purified by at least a high-temperature crystallization. It is taught that there is a synergy between the adsorption and the high-temperature crystallization, due to the fact that the adsorption is a very efficient process for recovering paraxylene from xylenes and that the crystallization is a very efficient means for purifying paraxylene to a very high level, which constitutes an ideal link between the two technologies. Moreover, this process emphasizes the full advantage of a smaller isomerization loop due to an almost quantitative recovery by pass of the paraxylene into the adsorption step of the unit.
Another patent U.S. Pat. No. 5,329,060 teaches a process comprising a selective adsorption step of a charge of a mixture of xylenes followed by a double-stage crystallization of paraxylene, one at very low temperature (-50 to -70.degree. C.) and the other at high temperature (0 to -10.degree. C.). The operating and investment cost of this process is all the higher since it provides a crystallization at very low temperature and a complete melting of the crystals obtained before their recrystallization.
Moreover, in the sequence of steps comprising an adsorption, a crystallization and especially an isomerization, to obtain very pure paraxylene, various types of impurities can appear in the various effluents and cause disturbances in the operation of units thus interfering with the yield obtained and the purity of the paraxylene recovered.
First, during isomerization of the paraxylene-depleted fraction, olefinic hydrocarbons can be produced in a variable amount depending on the values of the partial pressures of hydrogen introduced. The subsequent formation of polymers before and/or in the adsorption unit can cause serious problems with circulation through the adsorbent, and even destroy the adsorbent. Moreover, paraffinic and naphthenic hydrocarbons with 8 and 9 carbon atoms, whose-volatility is between that of, a desorption solvent, for toluene, and that of the xylenes, are intermediate products of the conversion of ethylbenzene into xylenes during the isomerization and their accumulation can prove to be harmful. Furthermore, aromatic hydrocarbons with 9 carbon atoms and more, present in small proportion and poorly separated in distillation columns, can be detrimental to the process, just like aldehydes and ketones that are heavier than the initial charge, which are formed when oxygen is accidentally dissolved.
Finally, another problem is linked to the presence of methanol. This alcohol is at times added in small proportion in mixtures of xylenes to be crystallized to prevent the co-crystallization of water and of paraxylene. Actually, the mixtures of dry C.sub.8 -aromatics are particularly hygroscopic and when the suspension of paraxylene crystals in the mother liquor passes into the centrifuge, water contained in the ambient air can be absorbed in the mother liquor and this water can possibly crystallize in connection with the temperature of this mother liquor. Moreover, some exchangers can have leaks and water can pass accidentally into the mixture to be crystallized.